Speed ratio of a V-belt continuously variable transmission and a toroidal continuously variable transmission for a vehicle is generally controlled by an electronic controller via an actuator which is responsive to a command signal output by the controller. A step motor is an example of such an actuator.
Tokkai Hei 8-178063 published by the Japanese Patent Office in 1996 discloses an initializing method of the command signal so as to make the command signal coincide with the actual operation position of the step motor. The operation position of the step motor is considered to be equivalent to the real speed ratio of the transmission.
The controller according to this prior art first determines if the vehicle is running, immediately after the power supply of the controller is started. If the vehicle is not running, the controller drives the step motor to an end position in the speed ratio increase direction which is equivalent to the maximum speed ratio of the transmission. At this position, the controller initializes the command signal such that a signal to drive the step motor to this position corresponds to a signal commanding the maximum speed ratio.
It is also possible that the power supply to the controller instantaneously stops or the voltage becomes too low for the operation of the controller while the vehicle is running. On such an occasion, if the step motor is driven to the end position when the power supply is recovered, the speed ratio suddenly takes a maximum value and an undesirable downshift of the transmission occurs. When the vehicle is running, therefore, the controller initializes the command signal by a different method. That is, the current operation position of the step motor is estimated from the real speed ratio of the transmission as detected by sensors, and the command signal is modulated to coincide with the estimated operation position of the step motor.